1. Field of the invention
The present invention relates to improved closed cycle refrigeration systems, particularly for removing excessive hydrocarbons or other chemicals from air-vapor mixtures by condensation.
2. Related information
Environmental concerns have led to a requirement for the reduction of chemical vapors, particularly hydrocarbons emitted into the atmosphere from every possible source. One of the sources targeted are bulk loading and unloading terminals. In these terminals a vapor recovery system is used to gather the air containing hydrocarbons or other vapors which are emitted during loading and unloading operations. The vapors are commonly separated from the air by refrigeration of the air-vapor mixture to cryogenic temperatures. The refrigeration systems can reduce "condensable" hydrocarbons or other condensable materials in the vented air to acceptable levels, i.e., loss than 80 mg/l.
The cryogenic temperatures required, i.e., about -100.degree. F., necessitate efficient refrigeration to achieve the separation. Since the total amount of refrigeration, expressed in tons (1 ton=12,000 BTU removed), is small when compared to large scale plants, such as ice plants and gas liquefaction plants, less efficient closed refrigeration cycles can be used with the common fluorocarbon refrigerants such as Freon 12.
With less efficient refrigerants, greater circulation and more compression power is required. Cascade refrigeration, using two refrigerants--one to cool the other--is an alternative. The most commonly used closed refrigeration cycle in these smaller scale plants is the compression of the refrigerant followed by cooling at the higher pressure and expansion across a Joules-Thomson expansion valve, the cycle being used to cool both refrigerants in the cascade refrigeration system. However, with the advent of concerns about damage to the atmosphere attributed to the fluorocarbons, these are being banned or expected to be banned in commercial operations.
A turboexpander may be used when the refrigerant is at a "free" higher pressure. The most common use of the turboexpander is in the natural gas processing industry. More specifically it is used in the demethanizer separation column when there is a free pressure drop, that is, the entering gas from the well or pipeline is at a pressure above that necessary for the separation. Generally the entering gas mixture is first pre-cooled by heat exchange with the exiting methane from the tower and the condensed liquids separated in a separator. The gas, containing mostly methane, is expanded across a turbine to the tower operating pressure and thus chilled. An enlarged top portion of the column acts as a separator for the expanded gas. To achieve an essentially isentropic expansion of the gas the turbine is coupled to a drive shaft which may have a brake applied or drive a compressor which partially compresses the product methane toward the desired pipeline pressure. The brake is required to vary the load on the turboexpander and match it with the compressor load. A situation where the mass flow in both the turboexpander and compressor is proportional is desirable for control purposes. Although the demethanizer is the most common application of the industrial turboexpander, its use in a closed refrigeration system has been proposed in "Introduction To Chemical Engineering Thermodynamics", Smith and Van Hess, Second Edition, McGraw-Hill Book Company, Inc. , New York, 1959, pp.301-303.
Adaptation of the turboexpander to the closed cycle system of refrigeration for smaller scale plants could achieve greater efficiency and allow use of less efficient refrigerants.